(1) Field of the Invention
The present invention relates to an aqueous urethane polyol having a urethane group, a hydroxyl group and a hydrophilic group altogether in a single molecule.
(2) Description of the Related Art
The aqueous coating is increasingly drawing public attention from the view point of global environment, safety and hygiene in recent years. It has come to be used for exterior coatings for building as well as industrial coatings for industrial products, such as for food cans and for coil coating. Further, there have been many proposals as to the one package automotive coating, which is required to have the high levels of quality, for example, in weatherablity, chemical resistance impact resistance and the like, as disclosed in, for example, GB 2,073,609, JP-A-63-175079 and GB 2,200,644. Most of the hardening agents used in these techniques are an alkyletherified melamine resin alone. A coating film formed by using a melamine resin as a hardening agent has superior properties, such as hardness, but is low in flexibility such as impact resistance.
In this connection, many proposals have been made to impart flexibility by introducing urethane bonds into a coating film. Methods for introducing urethane bonds include, for example, a method wherein an aqueous blocked polyisocyanate is used as a hardening agent and a method wherein a polyurethane dispersion is added.
As a technique to make a blocked polyisocyanate aqueous, for example, a surfactant is used for making a blocked polyisocyanate aqueous in U.S. Pat. No. 4,163,814. In U.S. Pat. No. 4,357,441, it is achieved by a method using a polyethylene oxide containing a higher aliphatic acid with 7 to 26 carbon atoms and blocking the isocyanate groups of a polyisocyanate with sodium bisulfite to obtain an aqueous blocked polyisocyanate.
JP-A-62-151419 and JP-A-2-3465 disclose a technique to make a blocked polyisocyanate with a isocyanurate structure, which is heat resistant, aqueous.
JP-A-62-151419 discloses a technique to impart water dispersibility to a blocked polyisocyanate to which polyoxyethylene groups are added, by further adding a water-soluble polymer compound, such as polyethylene glycol and a copolymer of ethylene oxide and propylene oxide.
In U.S. Pat. No. 4,925,885, hydrophilicity is imparted by using hydroxycarboxylic acid. The above-described blocked polyisocyanates tend to be inferior in low temperature hardening property, as compared with melamine type hardeners.
A technique relating to a polyurethane dispersion having urethane groups and urea groups within a molecule and has a number average molecular weight of above 20,000, is shown, for example, in U.S. Pat. No. 6,172,126. The polyurethane dispersion generally has no functional group and, thus, does not participate in a crosslinked structure. The coating film from a polyurethane dispersion, therefore, tends to have an inferior water resistance and further, a poor hiding power to smooth the unevenness of a substrate to be coated.
On the other hand, urethane polyols having both urethane groups and hydroxyl groups are known. Urethane polyols are utilized as a main component or a modifier therefor and can be utilized in versatile applications by using in combination with a hardening agent, such as a (block) polyisocyanate and a melamine type hardening agent. The examples of these proposals include polyester-polyurethane polyols obtained from an organic polyisocyanate and a polyester polyol in U.S. Pat. No. 4,485,228 and U.S. Pat. No. 4,540,766; urethane polyols obtained from a 2-5 functional polyisocyanate and a diol in U.S. Pat. No. 5,155,201 and U.S. Pat. No. 5,539,303; urethane polyols obtained from a 2-5 functional polyisocyanate and symmetric 1,3-diol in U.S. Pat. No. 5,130,405; urethane polyols with low viscosity in WO 96/40813; and a coating composition comprising a 2-5 functional cyclic polyurethane polyol in WO 98/16583. All of theses are, however, a non-aqueous type having no hydrophilic group and, thus, the development of an aqueous urethane polyol in consideration of the global environment has been awaited.